Two of the themes of Captains Courageous are the value of hard work and the process of maturation. When Harvey Cheyne first appears in the novel, he is portrayed as callow and spoiled: "His pasty yellow complexion did not show well on a person of his years, and his look was a mixture of irresolution, bravado, and very cheap smartness." Harvey has not been tested by real life, as he has lived a protected life, and, as a result, he is arrogant without reason to be so. His manner is rude and demeaning toward others, but he is not confident underneath his superficial bravado. For example, he tells the crew of We're Here, the boat that rescues him from drowning, "D'you suppose I'd fall overboard into your dirty little boat for fun?"

Later, hard work converts him into someone who is mature and who is truly confident. Kipling writes of Harvey: "He was a recognized part of the scheme of things on the We're Here; had his place at the table and among the bunks." By working hard along with the crew, Harvey earns his rightful place among them as a shipmate who offers something of value. In the process, he matures.

Another theme is tolerance. When Harvey is aboard the We're Here, he is introduced to people he would not have met in his earlier life. This includes the captain, Disko Troop and his son, Dan, and the Portuguese fisherman Manuel. Later, as different dories come together, Harvey meets even more fishermen:

As they drove into the confusion, boat banging boat, Harvey's ears tingled at the comments on his rowing. Every dialect from Labrador to Long Island, with Portuguese, Neapolitan, Lingua Franca, French, and Gaelic, with songs and shoutings and new oaths, rattled round him, and he seemed to be the butt of it all.

While Harvey felt superior to people from other backgrounds in his earlier life, when he is aboard the We're Here, he comes to know other kinds of people at sea and accepts them as equals. In the process, he grows in maturity and humanity.